Natural gas supply to residential or commercial customers requires reliable compact flowmeters with high accuracy, large flow metering range, and low pressure drop. Currently used displacement-type flowmeters are bulky and have moving mechanical parts which wear with time, resulting in drift of calibration.
Numerous flowmeters based on the fluidic oscillator concept have been developed in recent years. The earliest example of a liquid oscillator flowmeter is probably that of Wilson (1970), while more recent gas flowmeters have been described by Kawano (1986), Kalsi (1988), and Yasuda (1989). These fluidic flowmeters have been unsuitable for metering of residential natural gas flow because it has not been possible to combine the twin requirements of high rangeability and low pressure drop into a single device Kawano (1986) tried to overcome this limitation by using two fluidic flowmeters with overlapping range. However, this concept uses a mechanical valve to switch the flow from one flowmeter to the other, thus introducing a moving part which is susceptible to wear and failure. Yasuda (1989) combines his fluidic flowmeter with a semiconductor flowmeter to achieve the required rangeability with low pressure drop. Yasuda's fluidic flowmeter has a useful rangeability of only 20 and the semiconductor flowmeter has metering errors as large as 10 percent at the lower flow rate range.
Velocity measurement devices based on the thermal time-of-light (TOF) have previously been demonstrated by Bradbury (1971) for turbulent flow measurements and by Wesphal (1981) for measurement of wall shear stresses in gas flow boundary layers. Both of these devices have temperature sensors upstream as well as downstream of the pulse heated wire so as to measure flow direction as well as velocity. That implementation, however, prevents the measurement of gas velocities lower than about 0.3 m/s as reported by both authors. The ability to measure the very low gas velocities is critical in applying the TOF sensor to our low flow rate measurement range.
Needs exist for new flowmeters which are inexpensive and compact, which have no moving parts, which have high accuracies in measuring flow rates over varying ranges down to zero, and with low pressure drops.